Flat panel electron beam displays comprise a cathode and an anode contained in an evacuated envelope. In operation, the cathode is held at a negative potential relative to the anode. Electrons are emitted from the cathode. The potential difference between the cathode and the anode accelerates the emitted electrons from the cathode towards the anode. The emitted electrons are formed, within the display, into electron beams. A beam current thus flows between the anode and the cathode. In flat panel electron beam displays a matrix arrangement is disposed between the cathode and the anode. The matrix arrangement is formed by a pair of "combs" placed at right angles to each other. These are commonly referred to as rows and columns. Each pixel or subpixel lies at the intersection of a row and a column. Each of the combs has many separate elements (rows or columns). In operation, a control voltage is applied to each element of each of the combs. The control voltage applied to each element imposes an electrostatic force on the electron beam associated with that element. The electron beam current associated with that element can be adjusted by adjusting the control voltage.
Matrix driven flat CRT displays require the use of an area cathode to provide a uniform source of electrons to each pixel aperture. Field emission electron sources such as Metal-Insulator-Metal (MIM), Printable Field Emitter (PFE) and Field Emission Devices (FED) do not require heating, but are non space charge limited and suffer from problems of uniformity and instability that require some form of smoothing to make their use practical.
Thermionic cathodes are excellent sources of electrons. Thermionic remote virtual cathodes are known in the prior art. They form a uniform planar space charge cloud remote from the hot filaments, but these have problems of sensitivity to constructional tolerances, to ageing of the oxide cathodes and to voltage variations on control grids.